The biochemistry of the site-specific recombination that integrates the genome or bacteriophage lambda into that of its E. coli host has been studied. Supertwisted DNA has been discovered to be the obligate substrate for this reaction in cell free extracts. The supertwists are introduced into circular DNA by the action of a newly discovered enzyme, DNA gyrase. This enzyme has been purified and characterized; its role in recombination appears to be the same in whole cells as in the cell free system. An additional recombination enzyme, the product of the viral int gene, has been purified and characterized. It demonstrates specific binding to substrate DNA and, together with several bacterial proteins, carries out the breakage and reunion of DNA. BIBLIOGRAPHIC REFERENCES: Nash. H.A., Mizuuchi, K., Weisberg, R.A., Kikuchi, Y. and Gellert, M.: Integrative recombination of bacteriophage lambda - The biochemical approach to DNA insertions. In Shaprio, J., Bukhari, A. and Adhya, S. (Eds.): Plasmids, DNA Insertion Elements, and Episomes. New York, Cold Spring Harbor Laboratory, 1977, in press. Williams, J.G.K., Wulff, D.L. and Nash, H.A.: A mutant of Escherichia coli deficient in a host function required for phage lambda integration and excision. In Shapiro, J., Bukhari, A. and Adhya, S. (Eds.): Plasmids, DNA Insertion Elements, and Episomes. New York, Cold Spring Harbor Laboratory, 1977, in press.